July 3, 2015: As the mercury rose to record heights this week, I wonder how many of us gazed at the London skyline and thought about the quality of the air we breathe.

Back in the 1950s, the people of London could easily visualize the air pollution they created by burning too much fossil fuel. In fact, some residents couldn't even see their own feet one chilly December morning in 1952, as the dense smog known as a "pea-souper" rolled over the city.

Unusual winter weather patterns had caused the thick blanket of toxic fog in the sky to descend to street level, cutting off transport networks and leaving thousands of workers dead. This national tragedy is now known as the "The Great Smog".

Fast forward to the summer of 2015, and the blue skies are reassuringly clear, which makes it pretty hard to believe that air pollution claimed around 29,000 lives in the UK last year. The reason is that unlike the notorious "pea-soupers" of the 1950s, today's pollutants are largely invisible.

Nitrogen oxide, (NOx) and the ultra-small particulate matter (PM) commonly associated with diesel fuel emissions are prime suspects as researchers begin to unravel their effects on respiratory and cardiovascular health.

While a number of initiatives such as inner-city low emission zones have targeted motorists, the recent spike in popularity of diesel-fuelled cars is only part of the reason that the air we breathe is laden with these tiny, toxic particles.

A substantial source of air pollution comes from a place you have probably never thought about in your life. Welcome to the unfamiliar world of auxiliary engines.

One of the most common of these hidden polluters is the transport refrigeration unit (TRU), which is used on many of the trucks that move chilled produce to local cafés, food shops and even your home.

This auxiliary engine is tasked with keeping the precious cargo cold – but the alarming fact is that most run on diesel and release disproportionately high amounts of those toxic emissions.

This pollution is also more concentrated within cities, in the suburbs as well as the centre of town, because that is where you find most of the shops and restaurants receiving chilled goods.

The engines in question are quite small compared with modern diesel truck engines, so a bit like today's air pollution problem, they generally go unnoticed, which has allowed them to effectively slip under the radar of public health officials and the regulatory bodies who impose strict limits on the emissions of the cars we drive.

But that does not mean they are less polluting. In fact a transport refrigeration unit can emit up to 29 times more carcinogenic pollution than the larger main engine of the truck.

Translated across the whole UK fleet of roughly 84,000 TRUs, it starts to add up to be a big issue. In total the UK's TRUs could emit nearly 3 million kgs of NOx and more than 350,000 kgs of particulate matter every year. That's the same amount of particulate matter as is emitted in a year by around 5.5 million Euro6 diesel cars.

The UK use of these hidden polluters is also just the tip of the iceberg if you consider for a moment the fact that many developing countries are fast-tracking investment in refrigerated transport networks to support expanding middle class populations.

That growth will most likely see the numbers of refrigerated transport units globally more than double by 2025, with a projected 9.6 million vehicles on the road. In fact recent research has suggested that there could feasibly be as many as 15 million TRUs on the world's roads by 2025, which could have a major impact on both air quality and climate change.

Given that air pollution in the developing world is already estimated to be responsible for about 600,000 premature deaths per year in India, and 1.2 million in China, investing heavily in a diesel-fuelled transport refrigeration network could amount to nothing short of a global public-health crisis.

So why hasn't industry come up with a solution?

With no regulations governing emissions from transport refrigeration units in the UK or the EU, the truth is there has been little incentive for anyone to invest heavily in R&D. But more importantly, until now there hasn't been the game changing, and crucially cost-effective, technology available to enable a paradigm shift in the industry.

But for the sake of the air we breathe, we desperately need a viable, efficient and commercially sound alternative to diesel. With this problem in mind, Dearman has focused on developing a clean, zero-emission alternative.

We are not talking about the usual small incremental improvement, but revolutionary new technology.

At the heart of the Dearman transport refrigeration system is the Dearman engine (above), a British invention now in the final stages of full commercialization. The only emission from a Dearman engine is air and lots of cold – making it ideal for use in a transport refrigeration unit.

The engine runs on liquid nitrogen, which is available for immediate use almost anywhere in the UK and across most of the industrialized world.

What's more the Dearman system is expected to match the price of the current diesel units but be cheaper to operate and will be available to the industry in the very near future, with the first Dearman zero emission transport refrigeration unit set to hit the UKs streets this autumn.

If all transport refrigeration units in the UK were zero emission, it would be the equivalent of taking more than 5 million modern diesel cars off the road. Imagine what that could do to help clean up that 'invisible' pollution hanging around our city streets this summer.

- Tim Fox is a fellow of the Institution of Mechanical Engineers (IMechE) and joined Dearman in April this year to become the company's International Ambassador. Dearman is a technology company developing a portfolio of proprietary technologies, products and services that deliver reductions in operating cost, fuel usage and emissions at low capital cost.